Looking back at my old notes on setting this up (it's changed in the beta firmware(s) I'm currently running)
Can you try the following on the router:
1 = VPN and Remote Access >> IPsec General Setup
Enter a value in "Pre-Shared Key" and in the confirmation box and make a note of this - this is the "Secret" which I can see in your Linux setup dialog.
Save/OK (or whatever the option is to save)
2 = VPN and Remote Access >> IPsec Peer Identity
Index 1, Profile Name "Any", enble this account, check Accept Any Peer ID, everything else blank/unchecked.
Save/OK (or whatever the option is to save)
3 = VPN and Remote Access >> Remote DIal-in User
Now, check the "Specify Remote Node" option
...and in the "or Peer ID" box, enter the profile name created in step 2 ("Any")
Now you will be able to enter the "IKE Pre-Shared Key" you created in step 1 (and this is the "Secret" you need to tell Linux.
Save/OK (or whatever the option is to save)

My notes tell me that without the "Remote Node" being enabled on the Draytek, it was not possible to enter the IKE PSK.

Best of luck
Simon